Common Equity - Share Repurchase Program (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 01, 2013
Class of Stock [Line Items]
Common stock issued, Shares	0.0	0.0	0.0
Stock Repurchased During Period, Shares	1.8	1.5	2.7
Stock Repurchased During Period, Value	$ 108.0	$ 74.7	$ 123.2
Share Repurchase Plan, 2013
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount, Value				$ 300.0
Stock Repurchased During Period, Shares	0.0	0.0	0.4
Stock Repurchased During Period, Value	$ 0.0	$ 0.0	$ 18.6
Share Repurchases to Fulfill Exercised Stock Options and Restricted Stock Awards
Class of Stock [Line Items]
Stock Repurchased During Period, Shares	1.8	1.5	2.3
Stock Repurchased During Period, Value	$ 108.0	$ 74.7	$ 104.6